This invention relates to a method and apparatus for controlling the grasping of an object by a robotic hand. The Robot Industries Association defines an "end-effector" as, "an actuator, gripper, or mechanical device attached to the wrist of a manipulator by which objects can be grasped or otherwise acted upon." In this specification, the term "robot hand" or "hand" will be used to describe the subset of "end-effectors" that are used for grasping objects and which have some resemblance to the human hand. The term "robot" or "robot system" indicates the combination of the robot hand and a robot arm. As used in this specification, the term "acquisition envelope" is an imaginary volume attached to the robot hand and inside of which the object to be grasped must fit for successful grasping. The shape and dimensions of this imaginary volume are functions of the hand kinematics, the geometry of the grasped object, and the environment in which acquisition occurs. The present invention assumes that the robot arm has sufficient capabilities to position the "acquisition envelope" of the robot hand to surround the object to be grasped within the acquisition envelope, but the exact location of the object is not necessarily known.
The manipulation of objects, which are those material things perceptible by one or more of the senses such as vision or touch, is an important requirement for automated manufacturing activities such as assembly, machine loading/unloading and tool. Many of these tasks can be done by industrial robots having a mechanism for grasping, gripping and acquisition of objects.
One problem which limits the use of robots in such activities is the lack of generality that is associated with the hand designs. The requirement for robots to have a wide variety of robot hand designs results from the variety of physical characteristics of the object, such as size, weight, stiffness, fragility, etc. For example, many robot hands are designed to grasp particular objects, making these hands unsuitable for grasping other objects and resulting in custom-designed end-effectors for use with only one or two different objects. It has been estimated that such customized hands often represent 20 to 30 percent of the total robot system cost. Such lack of hand generality requires a new design for each activity or application. The resulting requirement for new hand designs with each change of manipulated object detracts from robot flexibility in many industrial manufacturing applications.
When a robot hand structure acquires or holds an object, the interaction force and moment between the hand and the object can be applied in a controlled or an uncontrolled mode. In an uncontrolled mode, the velocity and position of the closing hand is not controlled. Current industrial robot hand designs often do not control the velocity of hand closure and force application to an object, but rather apply forces in an uncontrolled or "impacting mode" to the object. The impact force is a dynamic force, which is approximately proportional to the contact velocity, i.e., the velocity of the hand member as that member contacts the object, and the square root of finger mass and equivalent stiffness. The contact velocity is sometimes called the impact velocity. Stiffness is the inverse of compliance and represents the ability of the object to store energy due to deformations from applied forces which occur during acquisition of the object by a robot hand. Such energy is not dissipated, but may create motion of the object or is returned to the hand while grasping the object.
When the relative velocity of the contacting implements of the hand is zero, the interaction force consists of a static force, which is the touch force between the hand implement and the object.
One way in which the control of hand-object force interactions has been traditionally accomplished has been through the use of compliant pads of foam or rubber on the fingertips to reduce contact forces, i.e., to dissipate the energy associated with the impact forces. This solution requires a variety of fingertip materials for acquisition of objects and also results in the loss of positioning accuracy for the highly compliant padded finger.
A second approach to limiting hand-object force interaction has been .by reducing the finger closure velocity of the hand by using reduced speed drives. This would correspond to using restrictions in the supply pressure to pneumatic drives. Such use of uniform finger velocity reduction to control the handobject acquisition forces usuallyreduces the gripping force which can be statically applied to the object for secure holding. Such finger velocity reduction also directly increases the acquisition cycle time, resulting in an undesirable hand design characteristic, namely unduly slow acquisition of an object.